bytes: rename _byteconcatLength to _byteconcatInteger

Signed-off-by: Daniel Thomas <drt24@cam.ac.uk>
Add toByteArray method to NigoriNonce

Signed-off-by: Daniel Thomas <drt24@cam.ac.uk>
bytes: add byteArrayToBigInteger

Signed-off-by: Daniel Thomas <drt24@cam.ac.uk>
bytes: use unsigned rather than signed bytes for conversion to List<int>

Signed-off-by: Daniel Thomas <drt24@cam.ac.uk>